1. Field of the Invention
The present invention relates generally to navigational apparatus, but more particularly to rangefinder/sextant apparatus having a combined search capability.
2. Description of the Prior Art
Radar is the standard and dominant method of determining range, azimuthal and elevational data. The data is used for search, navigational and weapons fire control purposes. However, radar has poor resolution. Objects appear as blips on its CRT face, and these blip presentations are subject to error from sea clutter making interpretation of information therefrom difficult. In addition, when radio silence is mandatory to prevent Fleet location, radar has to be shut down, and passive optical systems are required to substitute for radar for navigational and weapons fire control purposes. Optical imaging for some applications is superior to radar imaging. There are optical radar systems which provide simultaneous optical images and range information. But these active systems can be detected by infrared receivers. There is optical 360.degree. search systems in the prior art but they do not have a passive rangefinding capability or a sextant function capability.
As further background material, and incorporated herein by reference, U.S. Pat. No. 4,162,124, to Feldman et al., entitled, "Passive Optical Rangefinder - Sextant", and assigned to the same assignee as the present application, discloses a remotely controllable optical navigational instrument using rotatable index and stationary horizon mirrors in pairs asymmetrically disposed about a vertical plane bisecting the longitudinal axis of a stabilized horizontal imaging camera. The axes of rotation of the index mirrors are mutually normal and perpendicular to the longitudinal axis. When fixed at infinity, the index mirrors convey a double image of distant objects, via their respective horizon mirrors, to a spherical reflector and onto the imaging camera. Cooperating electronic circuitry selectively filters and scans the double image to determine the range of the object. Independently, the index mirror, rotatable about the horizontal axis, serves to measure altitude. The index mirror rotatable about the vertical axis serves to measure horizontal angles.
The present invention adds 360 degrees azimuthal and at least -5.degree. to about 60.degree. elevational search capabilities to the above mentioned apparatus of Feldman et al. In addition, the apparatus, according to the present invention, enhances the operational aspects of the remote sextant portion of the Feldman et al apparatus by allowing pointing of a search mirror at a navigational star in a direction which differs from that of an imaging camera which can be independently pointed at the sea horizon that is the least obscured by haze and/or sea clutter, thereby allowing altitude readings under adverse conditions.
The prior art, as indicated hereinabove, include advances in passive optical rangefinder/sextant apparatus, including remotely controllable index mirrors; however, in so far as can be determined, no prior art device incorporates all of the features and advantages of the present invention.